Modify nodes in compositing nodetree using Python
This bug report: https://developer.blender.org/T35720 pointed me to right right data path. When you want to modify node tree in compositor, then you have to use following path:

bpy.data.scenes['Scene'].node_tree.nodes['Alpha Over'].premul


or you can use following syntax to avoid hard-coded name of scene (current scene will be used):

bpy.context.scene.node_tree.nodes.['Alpha Over'].premul


and it just works. :-)